1. Field of the Invention
The present invention relates to a polishing slurry and a method of reclaiming wafers, and in particular to a chemical mechanical polishing slurry suitable for polishing of silicon wafers and polishing and removal of the films formed on wafers and to a method of reclaiming wafers using the same.
2. Description of the Related Art
Chemical mechanical polishing is a polishing by utilizing, in addition to a mechanical action of a polishing powder, a chemical reaction between the polishing powder and a wafer or between a chemical component in a polishing slurry and a wafer. The chemical mechanical polishing is characteristic in that it allows polishing of hard and brittle materials, allows polishing substantially without forming processing-degenerated layers, provides a smooth mirror surface, and has a higher processing efficiency. The polishing slurry is an important element for the chemical mechanical polishing having such characteristics.
A typical example of an application of the chemical mechanical polishing is polishing of silicon bare wafers. The slurry of colloidal silica dispersed in an aqueous alkaline solution has been used for polishing of silicon wafers. A reason for the use of alkaline colloidal silica is its ability of polishing the wafer surface to an undisturbed mirror surface (see, e.g., U.S. Pat. Nos. 3,170,273 and 3,328,141). The alkaline colloidal silica has been applied not only for polishing of silicon wafers but also for polishing and removal of a non-metal film such as polysilicon film, silicon oxide film and silicon nitride film formed on the wafers.
In the polishing of silicon wafers, a prevention of metal contamination is important. For example, it was shown that when a silicon wafer was polished with a slurry contaminated with Cu, Cu was not only adsorbed on the wafer surface but also penetrated into the wafer (see, Helene Prigge, Peter O. Hahn, Anton Schnegg, and Herbert Jacob, J. Electrochem. Soc., vol. 138, no. 5, pp. 1385-1389 (1991)). Thus, as a polishing slurry for effectively preventing the silicon wafer from the metal contamination, in particular contamination by transition metals such as Cu, Fe, Ni, Cr, and the like, a polishing composition containing a silicon oxide such as colloidal silica or fumed silica, one selected from the group consisting of inorganic salts of alkali metals, ammonium salts, cyclic amines and ethylenediamine, and a carboxylic acid having four to eight carboxyl groups in the molecule was proposed (see Japanese Unexamined Patent Publication No. 2002-226836 and U.S. Patent No. 2002/0151252 A1).
As it is widely understood, polishing of silicon wafers and polishing and removal of the non-metal films formed on wafers are preferably carried out under an alkaline condition. For that reason, the polishing slurry has been usually made alkaline by addition of a pH adjuster. As the pH adjuster, an alkali metal hydroxide such as NaOH or KOH or an alkali metal carbonate such as Na2CO3 or K2CO3 has been used. However, these pH adjusters, which contain metal ions, have been considered to be unsuitable for use in polishing in such a process of producing a semiconductor device that demands the protection from the metal contamination. Thus, as the pH adjuster containing no metals, organic amine compounds such as alkanolamines, ethylenediamine and the like and ammonia, have recently been used. The organic amine compounds and ammonia, which do not contain metal ions, are considered to serve to avoid a risk of the metal contamination of wafers.
However, even when the pH of the polishing slurry was adjusted with these compounds, if the polishing slurry was contaminated with Cu, it became apparent that the polishing caused also the Cu penetration into the wafer. For this phenomenon, proposed was a mechanism that Cu first forms a planer tetra-coordinated complex with the organic amine compound or ammonia, the resulting complex is adsorbed on the wafer surface, and thus the Cu is incorporated into the wafer (see, Helene Prigge, Peter O. Hahn, Anton Schnegg, and Herbert Jacob, J. Electrochem. Soc., vol. 138, no. 5, pp. 1385-1389 (1991)). Therefore, even when an organic amine compound or ammonia is used as the pH adjuster, it is quite difficult to avoid the metal contamination of the wafer.
Another example of the application of chemical mechanical polishing is planarization of the multilayer wiring in the process producing the semiconductor device. It is important in the CMP (Chemical Mechanical Polishing) to polish a particular film more efficiently than other films, and many polishing slurries were developed for improving the selectivity of the film removal. For example, in a case of the planarization of Al wiring, an acidic slurry containing an aluminum oxide polishing powder and an oxidizer such as hydrogen peroxide or iron nitrate is used for the purpose of leaving substantially all silicon oxide film that becomes an insulation layer. Alternatively, an alkaline slurry containing fumed silica and an alkali metal hydroxide or ammonia is used for polishing of an insulation film, for example a silicon oxide film.
In addition, the chemical mechanical polishing is used alone or in combination with other chemical etching in reclamation of used test wafers by removal of the films formed thereon. The “used test wafer” means a wafer used in the test for confirming whether the process producing the semiconductor device is adequate, and typical examples thereof include wafers having several layers of films formed on the surface for measurement of film thickness, wafers ejected as defective products from the process producing the semiconductor, and the like.
Use of a mechanical film-removing method such as wrapping or grinding for wafer reclamation results in generation of a processing-degenerated layer having a greater irregularity on the wafer surface after film removal. As a result, the processing-degenerated layer demands additional chemical etching for its removal and increases the burden of the next mirror-polishing step.
In contrast, the chemical mechanical polishing is more advantageously applied to the polishing and removal of films in the reclamation of used test wafers, because, different from the mechanical film-removing method such as wrapping, grinding, or the like, it causes substantially no generation of the processing-degenerated layer on the wafer and reduces the burden of the mirror-polishing step following the film removal. It also leads to a reduction in the decrease of the thickness of wafers during reclamation and thus to an increase in the recycled use rate of the wafers.
In the wafer reclamation treatment, it is important to polish many kinds of films uniformly and simultaneously during removal of the films formed on wafer, which is different from the case in the planarization of multilayer wiring. Thus, as the polishing slurry for reclaiming wafer that enables not only polishing of silicon wafers but also polishing and removal of non-metal films such as polysilicon film, silicon oxide film and silicon nitride film formed on wafers simultaneously and efficiently, U.S. Pat. No. 6,451,696 B1 discloses, for example, a polishing slurry of pH 9 to 12 containing monoclinic zirconium oxide having a primary particle diameter of 30 to 1,000 nm and a crystallite size of 10 to 1,000 nm in an amount of 2 to 20 weight %, and a polishing slurry of pH 9 to 12 containing cerium oxide having a primary particle diameter of 10 to 2,000 nm and an average particle diameter of 30 to 5,000 nm in an amount of 1 to 20 weight %.
Recently, Cu is used more frequently as a wiring material in the process producing the semiconductor device, and thus, the number of used test wafers carrying a Cu film formed thereon is gradually increasing. Consequently, the possibility of mixing of wafers carrying a Cu film in the wafer reclamation process is rising.
Conventional polishing slurries for wafer reclamation, which were developed mainly by focusing on improvement in the film-removing efficiency, were not effective in avoiding the Cu contamination. For example, the polishing slurry disclosed in U.S. Pat. No. 6,451,696 B1 contains an alkali metal hydroxide, an alkali metal carbonate, ammonia, an organic amine compound, or the like as the pH adjuster. Accordingly, when a used test wafer carrying a Cu film is subjected to film polishing and removal by chemical mechanical polishing using such the polishing slurry, Cu may penetrate into the wafer inside after film removal. In addition, all wafers reclaimed may become undesirably contaminated with Cu in the film-removing step.
Thus, there was no polishing slurry that allows polishing of silicon wafers and polishing and removal of various non-metal films formed on wafers simultaneously and efficiently and prevents effectively the metal contamination, in particular Cu contamination, of the wafers during polishing.